Major Events in Brain Development
Brain and spinal cord development begins with formation of the neural tube closure, which is completed by 28 days of gestation. Prosencephalization then occurs to form the cerebral hemispheres and diencephalon. At 2 to 4 months gestation, there is neuronal proliferation followed by migration at 3 to 5 months. Formation of the gyri begins with the major fissures being formed in the 5th month, the secondary sulci by 7 months, and the tertiary sulci by 9 months gestation. Finally, neuronal organization and myelination starts in the 3rd trimester and continues on well after birth. Organization and myelination are the major processes that account for brain growth and maturation.

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Neuronal Growth and Organization
By 36 weeks gestation, the process of neuronal proliferation is nearly complete. Although there is some limited proliferation of neurons throughout life, their contribution to brain growth is negligible. Growth of the individual neurons plus the elaboration and proliferation of dendritic and axonal processes and connections is responsible for most of the brain’s growth.
The diameter of a neuroblast is 5 microns while a mature neuron can measure 50 microns, a 1,000-fold increase in cell volume. Increase in the size of glial cells also occurs. During the organization process of brain development there is increasing dendritic arborization, synaptogenesis, and axonal connections, which make a significant contribution to brain growth as well as the maturation and development of brain function.

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Myelination and Development
The next developmental process that greatly contributes to brain growth and maturation is myelination. Myelination begins early in the 3rd trimester with the most rapid period of myelination occurring in the first two years of life. The myelination process follows a specific time course and pattern. The acquisition of neurodevelopmental milestones parallels and reflects this pattern. Myelination occurs early for motor-sensory roots, special senses and the brainstem; those structures necessary for reflex behavior and survival.
The corticospinal tract starts to myelinate at 36 weeks gestation and myelination is completed by the end of the 2nd year of life. Myelination of the corticospinal tract begins at the proximal portion of the axon and the shortest axons are the first to myelinate. The axons for the upper extremities and the trunk myelinate next. The axons for the lower extremities, which are the longest axons, are the last to myelinate with the process being completed by 24 months of age. This myelination pattern correlates with the progressive head-to-toe acquisition of developmental milestones.
The motor patterns of the immature brain are predominately under brainstem control and lack cortical modulation until there is the necessary maturation of these higher brain center connections.

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Cerebral Hemisphere Myelination
The areas of the cerebral hemispheres that are first to myelinate are the posterior portion of the frontal lobes, the parietal lobes and areas of the occipital lobes. The frontal and temporal lobes then myelinate and by the end of the second year, myelination of the cerebrum is largely completed, although there are interconnections of the association cortex that are still being myelinated into the 2nd and 3rd decades of life.

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